Using === NULL instead of is_null(), is actually useful in loaded server scenarios where you have hundreds or thousands of requests per second. Saving microseconds on a lot of "simple" operations in the entire PHP execution chain usually results in being able to serve more pages per second at the same speed, or lowering your cpu usage. People usually write very bad and slow code.

For the major quirky types/values is_null($var) obviously always returns the opposite of isset($var), and the notice clearly points out the faulty line with the is_null() statement. You might want to examine the return value of those functions in detail, but since both are specified to return boolean types there should be no doubt.

A second look into the PHP specs tells that is_null() checks whether a value is null or not. So, you may pass any VALUE to it, eg. the result of a function.isset() on the other hand is supposed to check for a VARIABLE's existence, which makes it a language construct rather than a function. Its sole porpuse lies in that checking. Passing anything else will result in an error.

Knowing that, allows us to draw the following unlikely conclusion:

isset() as a language construct is way faster, more reliable and powerful than is_null() and should be prefered over is_null(), except for when you're directly passing a function's result, which is considered bad programming practice anyways.

[Ed. note: this is because == tests for equivalence of value, but not type. NULL evaluates to
false, as does 0, so NULL == 0 is true--even though 0 is type int and NULL is type null.
You should use either is_null() as noted or ===, which returns true only if its operands are
equal and of the same type.]